The present invention relates to a data receiver used for a digital portable telephone set, a digital automobile telephone set, and the like.
It has been generally attempted in the digital portable telephone set, the digital automobile telephone set and so on to activate a receiving unit at a predetermined interval and deactivate it in the remaining intervals in order to reduce power consumption.
FIG. 1 shows a block diagram of a prior art data receiver. Numeral 21 denotes a demodulator for demodulating a received signal, and numeral 22 denotes a frame synchronization detector for detecting a specific bit sequence from the demodulated data to generate a timing signal to effect framing. FIG. 2 shows a structure of the demodulated received signal processed in such a data receiver. A synchronizing word or unique word 23 comprising the specific bit sequence which is used for synchronizing the received data and a data field 24 for storing data of voice or facsimile form one slot 25, and three slots 25a, 25b and 25c form one frame 26. The unique word 23 comprises 20 bits, the data field 24 comprises 260 bits and the frame length is of 20 ms.
The operation of the prior art is explained below. When the demodulator 21 receives a signal, it demodulates data by coherent demodulation, differential detection or others. The frame synchronization detector 22 calculates a correlation between the unique word of the demodulated data and a predetermined reference unique word, and if the correlation meets a predetermined condition, it starts a frame synchronization mode to process data frame by frame. During the frame processing, the correlation between the unique words is periodically calculated, every 3 slots in this case, and if the correlation is not maintained, it returns to a frame asynchronization mode. In the example shown in FIG. 2, when the first unique word 23a of the frame 26 is detected, the receiver is not activated during the rest period of the frame 26 and it is reactivated at the start of the next frame.
In the prior art data receiver, only when there is no error in the demodulated data from the demodulator and the specific bit sequence is periodically detected, the frame synchronization mode is started to receive the data for frame processing.
However, in the prior art data receiver, since errors may be included in the demodulated data from the demodulator 21 because of a waveform distortion created by a transmission line and a noise added to the data, the frame processing in the frame synchronization mode cannot be continued for a long period.